Having seen a fair share of BI tools (I created and maintain an "awesome" list at [1] which I created for the sole purpose of getting an overview of what's out there) and assessing the stage you're at developing solo, you're getting quite some important UI basics very right imo.
The "Next Steps" give a great impression of what's possible and how to navigate the app.
From an E-Commerce viewpoint, most people want to flexibly play around with Products, Orders and Order Line Items and ratios between the three. Configuring this by hand in something like Google Datastudio etc. is cumbersome, even or more so in Looker (if broken out into separate Views, one needs to needs to create Merged Results Looks; if not the different cardinality objects are always connected). I feel like Insights is handling this really well. If you would add support for calculated fields, you'd offer a compelling E-Commerce controlling solution.
One issue I noticed though is that your nesting seems to be unlimited. I could open an infinite "table" tree if I start from the order_lines or product view --> order_lines --> order --> order_lines --> product --> seller --> orders/ products... ad infinitum.
You want to make sure that this drilldown into FK relationships does not lead to loops. Also I think some visual support where in the nest tree one is at (orders --> order_lines --> product --> sellers) would be great, otherwise it's quite easy to get lost.
Other than that honestly congrats on doing a lot of things right in terms of data viz, responsiveness, sharing capabilities. Really like your work!
[1]: https://github.com/thenaturalist/awesome-business-intelligen...